1. Field of the Invention
The present invention relates to the art of electrochemical cells, and more particularly, to a novel electrode construction that improves the efficiency of a prismatic electrochemical cell by use of a full radius edge structure. This electrode shape is used in combination with a novel terminal lead/current collector connection to increase both the gravimetric and volumetric energy densities of the cell by occupying previously unused space within the casing.
2. Prior Art
The recent rapid development in small-sized electronic devices having various shape and size requirements necessitates comparably small-sized electrochemical cells of different designs that can be easily manufactured to power such electronic devices. These size and power considerations mean that the cell must have a high energy density, and one commonly used cell configuration for this purpose is a prismatic cell design.
A perspective view of the present state-of-the-art construction for a prismatic electrochemical cell 10 design is shown in FIGS. 1 to 3 and described in U.S. Pat. No. 5,250,373 to Muffoletto et al., which is assigned to the assignee of the present invention and incorporated herein by reference. Such a cell includes a casing 12 having spaced-apart generally planar front and back side walls 14 and 16 joined by curved end walls 18 and 20 which curve into a bottom wall 22 that curves up to meet the side walls 14, 16. The open top of casing 12 is closed by a lid 24. The casing 12 houses a cathode electrode, generally designated 26 (FIG. 3), in electrical association with an anode electrode, generally designated 34. Cathode electrode 26 comprises plates 28, 30 contacting a cathode current collector 32 to provide the cathode having a peripheral edge 35 normal to the plane of the opposed major surfaces thereof.
The anode electrode 34 for the prismatic cell 10 is comprised of anode plates 36 and 38 contacting corresponding wing sections 40 and 42, respectively, joined by the intermediate web section 44 of an anode current collector. A separator 46 is provided between the cathode electrode 26 and the anode electrode 34 to prevent direct physical contact between them. Shielding and insulating sheets, generally referred to as numeral 48, are also provided between the web section 44 of the anode current collector and the cathode electrode 26. The casing 12 and lid 24 are in electrical contact with the anode electrode 34 and serve as the anode terminal for the case-negative electrochemical cell 10. The cathode terminal is provided by a lead 50 which is electrically insulated from lid 24 and casing 12 by a glass-to-metal seal 52. The distal end of a tab 54 extending outwardly from the cathode current collector 32 provides a coupling element 56 comprising a tube which receives the inner end of the cathode terminal lead 50 therein.
To construct an anode-cathode subassembly according to the prior art, the cathode plates 28, 30 are pressed together and bonded against the current collector 32. Separately, the anode plates 36, 38 are pressure bonded to the corresponding anode wing sections 40, 42 and the web section 44 of the anode current collector is welded to the lid 24, which together with the cell casing provides the negative terminal for the cell. The shielding and insulating sheets 48 are then disposed between the web section 44 and the cathode electrode 26 heat sealed into the separator 46. The cathode terminal lead 50, electrically insulated from the lid 24 by the glass-to-metal seal 52, is then spot welded to the tube 56 to provide the positive cell terminal. This construction only provides a single spot weld at the open end thereof for connection to lead 50. It is desirable to provide a more robust construction for the positive lead connected to the cathode current collector.
The prior art prismatic cell 10 is completed by folding the anode wing sections 40, 42 with the associated anode lithium elements 36, 38 relative to the web section 44 and toward each other to position the anode elements 36, 38 in operative contact with the oppositely directed surfaces of the cathode electrode 26 having the separator 46 disposed between the anode and cathode electrodes. The cell is then activated by filling the casing 12 with an electrolyte 58 through an opening 60 provided in lid 24 after the cell components have been assembled therein and lid 24 has been welded to casing 12. In its fully assembled condition shown in FIG. 1, a closure means 62 is hermetically sealed in opening 60 to close the cell. The closure 62 is insulated from the interior of the cell by an insulator structure 64 (FIG. 2).